The Newsletter Popup WordPress plugin through 1.2 does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup)
https://wpscan.com/vulnerability/10eb712a-d9c3-46c9-be6a-02811396fae8/